It is frequently necessary to measure the forces acting upon a particular structure. Examples include measuring forces induced in objects while they are being acted on by manufacturing processes such as cutting or assembly, or forces exerted by normal or handicapped people in the course of their activities. Usually, the forces acting upon the structure are expressed in terms of a three-dimensional coordinate system, and may be completely expressed in terms of six components, including three linear force components along each of three mutually orthogonal axes and three torque components about the same set of axes. Devices for measuring such a set of six forces are termed "six degree of freedom sensors."
The conventional approach to six degree of freedom force sensor design is to use a structure having various hinges, pivots, or other mechanisms so that the components of the forces applied to the structure are decoupled by the action of the structure itself with the result that each of the six components of applied force may be measured independently of the other forces applied to the structure by one or more strain gauges appropriately located on the structure. Due to the difficulty of constructing mechanical structures which completely decouple the different forces, and since this approach requires knowing all of the dimensions and elasticities of the structure to an extremely high degree of accuracy, the cost and complexities of these types of structures tends to be high, and accurate measurements are difficult to obtain from them. Typically, force sensors of these types, as exemplified by U.S. Pat. Nos. 3,613,443 and 3,618,376, are made up of complicated mechanical structures requiring a very large number of strain gauges, 20 to 40 strain gauges being typical. Alternatively, specialized sensors having individual outputs for forces measured along each of three orthogonal axes may be employed, as shown in U.S. Pat. No. 3,640,130.